Tungsten oxide thin films are widely used as dielectric materials used for electronic devices such as capacitors, filters, and semiconductor chips, and the like, optical element materials used for optical communication filters, isolators, and the like, electrochromic materials used for light control mirrors, and the like, and gas chromic materials used for gas sensors, and the like. Moreover, tungsten oxide is known to function as a visible light responsive photocatalyst material, and is attracting high attention in view of industrial application. Conventionally, the tungsten oxide thin films have been formed by a vacuum deposition method, a sputtering method, a laser ablation method, a sol gel method, and the like.
JP-A 2001-152130 (KOKAI) describes a photocatalyst material obtained by sputter-depositing tungsten oxide on a substrate, where tungsten oxide having a triclinic crystal structure is mainly used. The sputter deposition exposes a substrate to high temperatures, and thus it may be not applicable depending on the heat resistant temperature of the substrate. The sputter deposition is complicated in process management or the like, and not only it becomes costly depending on the shape and size of the substrate, but deposition on a wide area of a building material or the like is difficult. A visible light responsive photocatalyst layer formed of a sputter-deposited tungsten oxide film is excellent in hydrophilicity, but has a problem that its decomposing performance for harmful gas of acetaldehyde and the like is insufficient. Hydrophilicity data under irradiation with visible light are not provided, and thus it is presumed that sufficient photocatalytic performance under visible light is not obtained.
Similarly to the sputtering method, the laser ablation method needs to heat a substrate to high temperatures for controlling crystallinity, and it may be not applicable depending on the heat resistant temperature of the substrate. Not only it becomes costly depending on the shape and size of the substrate, but deposition on a wide area of a building material or the like is difficult. Deposition of tungsten oxide by the sol gel method can be performed relatively inexpensively, but appropriate heating is needed for forming a tungsten oxide film with good crystallinity. Thus, it has constraints on heat resistant temperature of the substrate, shape of the substrate, and so on. When a tungsten oxide film is used as a photocatalyst film which decomposes organic gas or the like, it needs to have an appropriate crystal structure and a large specific surface area, but the sol gel method is not capable of controlling the crystal structure and the specific surface area of the tungsten oxide sufficiently.
A photocatalyst film using a titanium oxide or the like is generally formed by mixing a dispersion liquid obtained by dispersing titanium oxide particles in a dispersion medium with an inorganic binder to prepare a coating material, and applying this coating material on a substrate. The coating material can be applied on various substrates and further can be deposited at around room temperature by selecting a binder, and thus it is possible that the photocatalyst film is applicable to a wide range of products. In the case of a coating material using a dispersion liquid of a photocatalyst powder, it is necessary that particles are well dispersed in the film, for securing performance of the photocatalyst and obtaining film properties of high strength and smoothness. As a typical method to obtain a good dispersion coating material, there is known a method to produce a dispersion liquid in which a photocatalyst powder is dispersed sufficiently in a dispersion medium, and add a binder therein.
Aggregation can easily occur when ultra-fine particles are used like the titanium oxide based photocatalyst, and it is needed to add a finishing agent or a dispersion agent for stabilizing the dispersion state. However, the finishing agent and the dispersion agent are factors to limit photocatalytic performance, and efforts are made to decrease the amount of adding them as much as possible. For example, stabilization of the dispersion state by adding acid and decreasing pH is practiced. Further, lately, there are demands to decrease use amount of volatile organic compounds (VOC) in view of environmental protection, and aqueous dispersion liquids and coating materials using no organic solvent are increasing. Numerous non-ionic compounds are used as a dispersion agent in the aqueous dispersion liquids and coating materials.
When a film using the tungsten oxide powder is formed, first it is necessary to generate fine tungsten oxide particles. As a method of producing a fine tungsten oxide powder, there is known a method to heat an ammonium paratungstate (APT) in the air to obtain a tungsten trioxide powder (see JP-A 2002-293544 (KOKAI)). By the method heating APT in the air, a triclinic tungsten trioxide powder with a primary particle diameter of approximately 0.01 (BET specific surface area=82 m2/g) is obtained. For improving photocatalytic performance of the tungsten trioxide (WO3) powder, it is needed to be in a stable particle state.
By applying disintegration processing, the tungsten trioxide powder can be refined to a certain degree, but it is difficult to have a particle diameter of 100 nm or smaller for example, including aggregated particles. Moreover, when the disintegration processing is applied to make a fine powder, the crystal structure of a fine tungsten trioxide (WO3) powder changes by the stress of the disintegration processing. There occurs a defect to cause re-coupling of electrons and positive holes by stress of the disintegration processing, and thus using it as a photocatalyst conceivably causes decrease in performance. In the production method described in JP-A2002-293544 (KOKAI), kneading of 20 hours or more is needed for stabilizing a BET specific surface area, which poses a problem of low production efficiency of the tungsten trioxide powder.
As a method for obtaining a fine powder efficiently, thermal plasma processing is described in JP-A2006-102737 (KOKAI) for example. By applying the thermal plasma processing, a fine powder with a particle diameter of 1 nm to 200 nm is obtained. Besides the thermal plasma processing, as processing methods capable of oxidizing a tungsten material while sublimating it in an oxygen atmosphere, there are known arc discharge processing, laser processing, electron ray processing, gas burner processing, and the like. However, when a fine tungsten oxide powder produced by applying these methods is used as it is as a photocatalyst, there may be a case where optical characteristics and crystal structure are not optimum, and sufficient photocatalyst characteristics cannot always be obtained. Therefore, for forming a photocatalyst film, it is necessary to perform control to have appropriate optical characteristics, crystal structure, and particle diameter in a powder state.
For forming a stable dispersion liquid using the tungsten oxide particles, it is important not only to use tungsten oxide particles with a fine primary particle diameter, but also to facilitate decomposition of, for example, aggregated particles and prevent re-aggregation. However, under the current situation, a dispersion liquid and a coating material in which the tungsten oxide particles do not separate or precipitate for a long period have not been obtained. Particularly, when the tungsten oxide particles are applied to a photocatalyst, the dispersion liquid and the finishing agent become a factor to limit photocatalytic performance. Thus, a small amount of the dispersion liquid or the finishing agent is added, or a dispersion liquid to which it is not necessary to add them is desired. However, sufficient performance has not been obtained with such a dispersion liquid or a coating material.